>>252485952 mil copies on Gamecube is actually fairly significant.
First of all, handhelds are prefered for Pokemon due to trading, making Colloseum a single-player add-on to the main games for most people (despite it being good on it's own) so making those sales as what amounts to DLC to R/S/E and FR/LG to the majority of players is crazy good.
Secondly, the console wars were not entirely kind to GC, and it sold less than most other Nintendo console, and colloseum still saw great sales.
In short, odds were against it and it still beat out the local competition.
